The present invention relates generally to optical scanners and, more particularly, to occluding aperture stop components of optical scanners having folded light paths.
Optical scanners are used to produce machine-readable data which is representative of a scanned object, e.g. a page of printed text. Optical scanners employ line-focus systems to image scanned objects.
In a line-focus system, a light beam from an illuminated line object is imaged by a lens on a linear photosensor array which is positioned remotely from the line object. The linear photosensor array is a single dimension array of photoelements which correspond to small area locations on the line object. These small area locations on the line object are commonly referred to as "picture elements" or "pixels." In response to light from its corresponding pixel location on the line object, each photosensor produces a data signal which is representative of the light intensity which it experiences. All of the photoelement data signals are received and processed by an appropriate data processing system. In an optical scanning device, the illuminated line object of the line-focus system is commonly referred to as a "scan line."
Optical scanners and various components thereof are disclosed in U.S. Pat. application Ser. No. 383,463, filed July 20, 1989, for OPTICAL SCANNER of David Wayne Boyd; U.S. Pat. No. 4,709,144 for COLOR IMAGER UTILIZING NOVEL TRICHROMATIC BEAM SPLITTER AND PHOTOSENSOR of Kent J. Vincent; and U.S. Pat. No. 4,870,268 for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS of Kent J. Vincent and Hans D. Neuman, which are each hereby specifically incorporated by reference for all that is disclosed therein.
A problem experienced in most line-focus systems is that the light intensity of the line image produced at the linear photosensor array is not uniformly proportional to the light intensity at the line object. Generally, if a line object is evenly illuminated across its length, the light intensity at the sensor is much brighter in the area corresponding to the center of the line object than in the areas corresponding to the ends of the line object. This effect is quite significant; the intensity at the line image center may be roughly twice the intensity as that at the ends. There are several different optical effects which produce this problem. These optical effects are due mainly to the different distances of the various points on the line object from the central axis of the lens.
It is known in the prior art relating to line-focus systems to utilize an aperture stop positioned in the light path between a line object and a photosensor to differentially occlude light in the light path extending between the line object and the photosensor. Such "occluding" or "compensating" aperture stops are designed to partially block a proportionally greater portion of the light at the center of the light path than at the ends so as to make the light intensity at the photosensor uniformly proportional to the light intensity at the line object. Methods for determining the appropriate shape for such an occluding aperture stop are disclosed in U.S. Pat. application Ser. No. 389,033 filed Aug. 3, 1989, for METHOD OF DETERMINING APERTURE SHAPE of David Wayne Boyd, which is hereby specifically incorporated by reference for all that it discloses.
In some optical scanners, such as described in U.S. Pat. application Ser. No. 389,033 referenced above, the scanner light source, the focusing lens assembly, and the optical sensor are all contained in a movable carriage assembly. The imaging light path extending between the scanned object and the scanner lens is generally "folded" by one or more mirrors in order to reduce the carriage size needed to accommodate the light path. The different folded portions of the light path are often positioned quite close together and in many cases overlap with adjacent light path portions along a significant portion of the light path length.
The imaging light path generally increases in thickness and decreases in width as it approaches the lens, thus the size of an occluding aperture stop which is to be provided in the scanner light path is dependent upon the position along the light path at which it is mounted. The relative effectiveness of an occluding aperture stop is, for a number of reasons, also dependent upon its position along the light path. An occluding aperture stop positioned next to the line object would theoretically be the most effective. However, at such a location the occluding aperture stop would be so small as to make fabrication and accurate positioning extremely difficult, and thus such an aperture position is generally not practical. An aperture stop positioned next to the scanner lens is the least effective position and is thus not desirable. As a result, it is usually desirable to position an occluding aperture stop at a mid-region of the light path. However, in scanners having folded light paths, it is often difficult or impossible, at a mid-region of the light path, to position an occluding aperture stop in one folded light path portion without causing the aperture stop to also extend into an adjacent folded light path portion.